Coburn, 65, has been suffering from a recurrence of prostate cancer and has hinted privately to reporters and in a published interview last week that his health troubles might force him to leave office before his current term expires in early 2017. He had already announced that he would not run for reelection, meaning his decision will have him exit two years sooner than previously planned.

In a statement Thursday evening, he said that he and his family have been "touched by the encouragement we've received from people across the state regarding my latest battle against cancer. But this decision isn't about my health, my prognosis or even my hopes and desires."

"As a citizen, I am now convinced that I can best serve my own children and grandchildren by shifting my focus elsewhere," he added. "In the meantime, I look forward to finishing this year strong."

Coburn's exit paves the way for a special election. As The Fix notes, he can tender his resignation early so that the state holds the special election on Election Day earlier this year. That would save the cost of holding a separate special election in 2015 -- something the thrifty Coburn would likely prefer -- and would allow for a successor to be seated immediately after Coburn's resignation date.

Coburn was one of two Republican senators who did not vote Thursday afternoon on the $1.1 trillion omnibus spending bill.

For most of the past 20 years, Coburn has represented the ideological moral high ground for conservatives. Believing in the role of a "citizen legislator," he imposed a three-term limit on his House service, and unlike many of his fellow classmates from the 1994 elections, he lived up to his pledge and left Congress after the 2000 elections.

After Sen. Don Nickles announced his retirement in 2004, Coburn won that seat while pledging to serve just two six-year terms. In his first term he was pretty clearly the most conservative member of the Senate, quickly learning how to use the arcane parliamentary procedures to his advantage in order to block legislation that he deemed wasteful no matter how popular it was with his colleagues. By his own estimate he saved the federal government billions of dollars in new government programs -- many of which would have passed with 90 votes or more.

Yet by 2011, after winning reelection to his final six-year term, Coburn found himself surrounded by many other self-proclaimed tea party conservatives. His appreciation for wing men in the conservative crusade was quickly followed by a disdain he felt toward what he considered their unwillingness to learn the Senate's procedures inside out.

Some saw a moderation in Coburn as he negotiated with a bipartisan "Gang of Six" senators in 2011 over a potentially massive $4 trillion debt package that would include major tax increases, prompting some outside conservative groups to question his loyalty to the cause.

The group's failure, and subsequent implosions of other bipartisan efforts at debt deals, turned Coburn into more openly dour legislator.

As he put it in his statement Thursday, and in private remarks to reporters recently, his diagnosis of the Senate's own gridlocked illness played as large a role as did his recurring bouts with cancer in his decision to plan his retirement.

He dropped hints of the potential seriousness of his health problems in an interview last week with Politico, where he admitted that a new round of chemotherapy treatment and an upcoming surgery might temporarily keep him from his Senate duties.

"I’m a straight shooter," Coburn said in the interview. "When I get ready to make a decision on what I’m going to do, I’m going to put it out there."

Coburn's early exit will trigger a special election to fill the remaining two years of his term, a race that is likely to draw interest from Oklahoma's five Republican House members: Reps. James Lankford, Tom Cole, Frank D. Lucas, Jim Bridenstine and Markwayne Mullin.

Read Coburn's full statement below:

“Carolyn and I have been touched by the encouragement we’ve received from people across the state regarding my latest battle against cancer. But this decision isn’t about my health, my prognosis or even my hopes and desires. My commitment to the people of Oklahoma has always been that I would serve no more than two terms. Our founders saw public service and politics as a calling rather than a career. That’s how I saw it when I first ran for office in 1994, and that’s how I still see it today. I believe it’s important to live under the laws I helped write, and even those I fought hard to block.

“As a citizen legislator, I am first and foremost a citizen who cares deeply about the kind of country we leave our children and grandchildren. As I have traveled across Oklahoma and our nation these past nine years, I have yet to meet a parent or grandparent who wouldn’t do anything within their power to secure the future for the next generation. That’s why I initially ran for office in 1994 and re-entered politics in 2004. I’m encouraged there are thousands of Americans with real-world experience and good judgment who feel just like I do. As dysfunctional as Washington is these days, change is still possible when ‘We the People’ get engaged, run for office themselves or make their voices heard. After all, how else could a country doctor from Muskogee with no political experience make it to Washington?

"As a citizen, I am now convinced that I can best serve my own children and grandchildren by shifting my focus elsewhere. In the meantime, I look forward to finishing this year strong. I intend to continue our fight for Oklahoma, and will do everything in my power to force the Senate to re-embrace its heritage of debate, deliberation and consensus as we face our many challenges ahead.

"May God bless you, our state and our country."

Aaron Blake contributed to this report.

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